According to the prior art, automatic transmissions, particularly for motor vehicles, comprise planetary gear sets that are shifted using friction elements or shift elements such as clutches and brakes, and typically are connected to a start-up element, such as a hydrodynamic torque converter or a fluid coupling, that is subject to a slip effect and is provided optionally with a lock-up clutch.
Such an automatic transmission is known, for example, from the applicant's patent DE 199 49 507 A1, according to which two non-shiftable front-mounted gear sets are provided on the drive shaft and generate two rotational speeds on the output side, which, in addition to the rotational speed of the drive shaft, can be selectively meshed with a shiftable double planetary gear set acting on the output shaft by selective engagement of the shift elements such that only one of the two currently actuated shift elements must be engaged or disengaged to shift from one gear to the next higher or lower gear.
By using five shift elements, seven forward gears are obtained; by using six shift elements, nine or ten forward gears are obtained.
Furthermore, patent DE 102 13 820 A1 makes known a multi-speed automatic transmission with eight forward gears and one reverse gear, which comprises a first input path T1 of a first transmission ratio; an input path T2 that has a higher transmission ratio than that of input path T1; a Ravigneaux type planetary gear set with four elements, namely, a first element, a second element, a third element and a fourth element in the sequence of elements in a rotational speed diagram; a clutch C-2 which transfers rotation of the input path T2 to the first element S3; a clutch C-1, which transfers rotation from the input path T2 to the fourth element S2; a clutch C-4, which transfers rotation from the input path T1 to the first element; a clutch C-3, which transfers rotation from the input path T1 to the second element C3; a brake B-1, which produces meshing of the fourth element; a brake B-2, which produces meshing of the second element; and an output element that is coupled to the third element S3.
Furthermore, a nine-speed multi-step transmission is known from DE 29 36 969 A1; it comprises eight shift elements and four gear sets, wherein one gear set serves as a front-mounted gear set and the main gearing includes a Simpson set and a further gear set serving as reverse gearing.
Further multi-step transmissions are known, for example, from the applicant's patents DE 102005010210 A1 and DE 102006006637 A1.
Automatically shiftable vehicle transmissions, of the planetary design, are already generally described numerous times in the prior art and are continually undergoing further development and improvement. These transmissions should have a relatively simple design, in particular requiring a low number of shift elements, and minimize the need for double shifting when sequential shifting is performed, thereby ensuring that only one shift element is ever switched when shifting is performed in defined groups of gears.
The applicant's document, DE 102008000428.3 which is not yet published, discloses a multi-step transmission of a planetary design that includes an input shaft and an output shaft which are disposed in a housing. The known transmission includes at least four planetary gear sets which are designated, in the following, as the first, second, third, and fourth planetary gear set, at least eight rotatable shafts which are designated in the following as the drive shaft, output shaft, third, fourth, fifth, sixth, seventh, and eighth shaft, and at least six shift elements comprising brakes and clutches, whose selected engagement produces different transmission ratios between the input shaft and the output shaft, and therefore preferably nine forward gears and one reverse gear can be realized.
The first and second planetary gear sets, which are preferably designed as minus planetary gear sets, form a shiftable front-mounted gear set, wherein the third and fourth planetary gear sets form a main gear set.
In the known multi-step transmission, the carriers of the first and second planetary gear sets are coupled together via the fourth shaft which is connected to an element of the main gear set, the ring gear of the first planetary gear set is coupled to the sun gear of the second planetary gear set via the eighth shaft, which is detachably connectable to the drive shaft via the first clutch, and the sun gear of the first planetary gear set can be coupled to a housing of the transmission by means of the third shaft, via a first brake, and is detachably connectable to the drive shaft via a second clutch, wherein the ring gear of the second planetary gear set can be coupled to a housing of the transmission by means of the fifth shaft via a second brake. In addition, the seventh shaft is constantly connected to at least one element of the main gear set, and can be coupled to a housing of the transmission via a third brake, wherein the sixth shaft is constantly connected to at least one further element of the main gear set and is detachably connectable to the drive shaft, via a third clutch; the output shaft is constantly connected at least to one further element of the main gear set.
In the known transmission, the fourth shaft is preferably constantly connected to the ring gear of the third planetary gear set, wherein the sixth shaft is constantly connected to the ring gear of the fourth planetary gear set and to the carrier of the third planetary gear set, and is detachably connectable to the drive shaft via the third clutch. Furthermore, the seventh shaft is constantly connected to the sun gears of the third and fourth planetary gear sets, and can be coupled to a housing of the transmission via the third brake. In this case, the output drive is produced via the output shaft that is constantly connected to the carrier of the fourth planetary gear set. Furthermore, the third and fourth planetary gear sets can be combined or reduced to a Ravigneaux set having a common carrier and a common ring gear.